German Patent No. 1,129,894 (British Patent No. 885,762) describes a process for sealing and reinforcing geological formations against water or gas by means of polyurethanes, which process mainly consists in that the reactants which form a cross-linked product containing the group: --NH-CO-O, that is to say a diisocyanate or polyisocyanate and a polyol having at least three reactive OH groups, are mixed together in a liquid vehicle and the mixture is forced under pressure into the formation which is required to be sealed or reinforced.
The di- or poly-isocyanates recommended for this purpose are toluylene diisocyanate or prepolymers of toluylene diisocyanate and polyhydroxyl compounds, such as castor oil or hexane triol. These compounds have disadvantages as a result of which they are difficult or impossible to use below ground. Toluylene diisocyanate has a relatively high vapor pressure and the vapors are damaging to human mucous membranes, in particular those of the respiratory organs. Prepolymers of toluylene diisocyanate have a high viscosity and generally insufficient stability in storage. Almost without exception they must be processed together with liquid vehicles, such as solvents or plasticizers. These liquid vehicles lower the flashpoint of the mixtures and reduce the strength of the resulting polyurethanes.
To obviate the above-mentioned disadvantages of toluylene diisocyanate, it has become increasingly customary in recent years to produce polyurethane casting resins from polyisocyanates which, in addition to 4,4'-diisocyanatodiphenylmethane, contain mainly polyphenyl-polymethylenepolyisocyanates. These polyisocyanate mixtures, known in the international literature as "MDI," have the disadvantage, when used for reinforcing geological formations and loose rock and earth masses as mentioned, for example, in German Patent Application P 2,436,029.0 (German Auslegeschrift No. 2,436,029) and U.S. patent application Ser. No. 598,108, filed July 22, 1975 that they are incompatible with the conventional polyhydroxyl compounds, such as polyetherpolyols and castor oil (publication by Bayer AG "Baygal/Baymidur, Polyurethan-Giessharze," issued 1.11.1974, order no. KL 43006, page 15, column 1, last paragraph, according to which it is necessary to stir the MDI with the polyhydroxyl compound for three minutes, otherwise there is a risk of separation of the mixture into its components; see also G. Lottanti and K. Schiegg, Kunststoffe-Plastics Vol. 2, 1976, page 19, column 2, paragraph 3).
The customary method employed in the production of polyurethanes for overcoming the incompatibility of MDI with polyhydroxyl compounds by increasing the reactivity of the mixture by addition of catalyst, such as tertiary amines or metal compounds, cannot be used in the reinforcement of geological formations or loose rock and earth masses because once the polyisocyanate and polyhydroxyl compound have been mixed, there must still be a sufficient time available for the mixture to penetrate the cracks and pores of the formation before it solidifies or undergoes an unacceptable increase in viscosity. Even without the addition of the above-mentioned catalysts, polyisocyanatepolyol mixtures have in any case only a brief working life barely sufficient for reinforcing purposes (Stahl: Das Verfestigen des Kohlenbeins unterhalb der Kopfstracke in mannlosen Streben mit Polyurethanschaumen, Gluckauf, 104, (1968), no. 15, page 667, left-hand column, fourth paragraph).
The second chemical method available for reducing the incompatibility of MDI with polyols, which consists of first preparing prepolymers, is also unsuitable for reinforcing purposes because the viscosity of prepolymers is too high.
It has now surprisingly been found that the problems of incompatibility which arise when "MDI" is used as polyisocyanate component for the reinforcement of geological formations may be overcome by using, as the polyisocyanate component, a mixture which contains from about 10 to 80%, by weight, of 2,4'-diisocyanato-diphenylmethane. This finding is quite unexpected since, on the one hand, it has hitherto been customary to overcome incompatibility by increasing the reactivity (addition of catalyst), as mentioned above, and, on the other hand, it is known that 2,4'-diisocyanato-diphenylmethane is less reactive towards polyhydroxyl compounds than the corresponding p,p'-isomer (German Auslegeschrift No. 1,923,214, column 5, lines 63 et seq.; the same teaching is given in British Patent No. 1,263,439).